Portable sign retroreflectometers quantify the reflective behavior of retroreflective materials used to make symbols and backgrounds on highway signs. As shown in FIG. 1, a retroreflectometer simulates illumination 51 from the headlights 53 of a motor vehicle 55 via an appropriate light source and measures the amount of retroreflected light 57 that propagates from the sign 59 in the direction of the driver's eyes is retroreflected directly back toward the headlights 53, but since the angular distribution of the reflected light has some angular spread, some of the light is directed toward the drivers eyes.
Current American Society for Testing and Materials (ASTM) standards specify that the sign 59 be illuminated by light situated at an illumination angle A1 of four degrees, and that the retroreflected light be measured at an observation angle A2 of 0.2 degrees above the retroreflected lobe which is directed back toward the headlights 53. Retroreflectometers which follow this standard simulate the angular separation between the headlights and a driver's eyes for a sign 150 to 200 meters in the distance. European standards require an illumination angle A1 of five degrees and an observation angle of 0.33 degrees.
In order to accomplish the above simulation, typical prior art retroreflectometers employ a single-element photodetector positioned relative to an illumination lamp such that the photodetector receives the retroreflected light at the observation angle. However, these devices have drawbacks. First, where the single-element detector is positioned behind or near the light source, it can be difficult to eliminate the light interference from the light source. Also, the angles of reflection should be very precise to ensure accurate measuring. This accuracy necessitates a bulky rigid structure to ensure a proper angle, etc.
In some cases, a beam splitter is used to separate the retroreflected light from the illumination light. The single-element detector is positioned to allow the retroreflected light to fall upon it. The retroreflected light incident to the single-element detector is typically round in shape, with the intensity being Gaussian in nature, with more intense light at the center and less intensity at the edges. In order to determine the intensity at the edges, prior art devices define an annulus which is centered at the center of the incident retroreflected light. The annulus is typically defined by placing a precise mask over the single-element detector that only allows the retroreflected light to pass through what falls in the annulus region which is defined by the observation angle as discussed previously. However, this configuration is not without its drawbacks.
Specifically, the angle of the splitter is precisely placed to ensure that the retroreflected light strikes the mask centered on the annulus. For example, if the single-element detector is separated from the beam splitter by 10 mm and the detector is 175 microns in diameter, then the angular position of the beam splitter is necessarily held to less than one milliradian and the detector positioned to within a tolerance of 17.5 microns in order that the required observation angle A2 of 0.2.degree. be maintained to within 20%. Such precision necessitates bulky structure to ensure that the splitter does not move, even though the unit is handled roughly. Note that the mask which define the annulus is employed in some prior art devices without a splitter. These devices also necessitate bulky and precise structure to ensure that component parts maintain precise positions for accurate readings. Thus, the cost of manufacturing retroreflectometers according to the prior art is undesirably high due to the need for precision component parts and assembly. Also, these devices lack flexibility due to the nature of the single element detector.
In light of the forgoing, there is a need for a retroreflectometer which can accurately measure the intensity of retroreflected light within a predetermined area without the need for expensive and bulky structure, providing maximum flexibility in the nature of the predetermined area.